This paper presents an associative behavior-based abundance index (ABBI) providing direct estimates of the abundance of yellowfin tuna (Thunnus albacares) based on their associative behavior around floating objects (FOBs). Considering the associative dynamics of small yellowfin tuna individuals (<10 kg) at FOBs through residence and absence times at FOBs, together with acoustic data obtained from fisher’s echosounder buoys, the ABBI index is derived for yellowfin tuna in the western Indian Ocean over the period 2013-2019. This index accounts for both the FOB-associated and free-swimming components of the tuna populations, as well as for the effects of increasing numbers of FOBs on each component.